The present invention relates to a fixing mechanism which fixes a toner image (image) on a fixation member in an image forming apparatus such as an electro-static process copying machine or a laser printer, and more particularly to a fixing mechanism using an induction heating method.
A fixing mechanism incorporated in a copying apparatus using an electrophotographic process heats and fuses toner which is a developer formed on a fixation member and fixes the toner on the fixation member. As a method for heating the toner which can be used in the fixing mechanism, a method using radiant heat from a halogen lamp is widely utilized.
As the method using the halogen lamp as a heat source, there is extensively used a structure in which a pair of rollers are provided so as to be capable of providing a predetermined pressure to the fixation member and the toner and a cylindrical halogen lamp is arranged in an inner space of at least one of the rollers as a hollow cylinder. In this structure, the roller having the halogen lamp arranged therein forms an action portion (nip) at a position where it is brought into contact with the other roller and provides the pressure and heat to the fixation member and the toner guided to the nip. That is, the fixation member, namely, paper is passed through a fixation point which is a pressure welding portion (nip) between a heating roller to which the lamp is provided and a pressure roller which rotates in accordance with the heating roller, and the toner on the paper is fused and then fixed onto the paper.
In the fixing mechanism using the halogen lamp, light and heat from the halogen lamp are radiated in the circumferential direction of the heating roller and the entire heating roller is heated. In this case, taking the loss when light is converted into heat and the efficiency or the like when warming the air in the roller and transmitting the heat to the roller into consideration, the heat exchange effectiveness is 60 to 70%. Further, it is known that the heat efficiency is low, the power consumption is large and the warming-up time is prolonged.
In order to solve the above-described problems inherent to the heater fixation, as shown in Jpn. Pat. Appln. KOKAI Publication No. 9-258586 or Jpn. Pat. Appln. KOKAI Publication No. 8-76620, there is proposed a fixing mechanism using a technique of induction heating.
Jpn. Pat. Appln. KOKAI Publication No. 9-258586 discloses a fixing mechanism which passes a current to an induction coil obtained by winding a coil around a core provided along a rotational axis of a fixing (metal) roller and generating an induction current to the roller in order to heat the metal roller itself.
Further, Jpn. Pat. Appln. KOKAI Publication No. 8-76620 discloses a fixing mechanism which has a conductive film accommodating therein magnetic field generating means and a pressure roller pressed against the conductive film and fixes the toner on a recording medium carried between the conductive film and the pressure roller onto the recording medium by causing the conductive film to generate heat.
For the purpose of reducing the warming-up time, in the fixing mechanism having a thinner heat roller or the fixing mechanism which adopts a belt or the like, a temperature hysteresis distribution is apt to appear on the heat roller depending on a size of the paper inserted and only a part through which the paper has passed consumes the heat energy. Therefore, irregularities in temperature are generated, and hence temperature control or a partial heating method which does not depend on the paper size is required in the fixing apparatus to which induction heating is applied in particular.